Today, a vital aspect of computers, tablets, and phones is that they are connected to each other. Computers can communicate with each other, and we can communicate via computers because they're connected in a network. A network is a number of computers connected together by links. If you draw them in a diagram like this and then zoom out, it can look a bit like a fishing net hence the term network. Traditionally, computers in a network would have been connected together using wires, normally copper cables like this one. Not too different from the cables used to carry out electricity or telephone conversations. Now, copper wire is increasingly being replaced by optic fibers, where signals are sent using light which is much faster than sending signals over copper. Or it's replaced by wireless networks where communication happens via electromagnetic waves that don't require computer to be physically connected to the network. Not all networking technology is created equal. It's quite obvious that a wireless network is different from a wired network, there's no wires. Wireless networks are more convenient in many ways, but there are other factors that can make a wired network better. One is bandwidth which is the amount of data that can be sent over the network within a given time. We commonly talk about how fast a network is and that refers to its bandwidth. High bandwidth networks are obviously better particularly if you're using a lot of data for example by streaming video. Wi-Fi networks are often has high bandwidth as many wired networks, but mobile networks can have fairly low bandwidth. Another factor in the performance of networks is reliability. How sure can we be that a message sent over the network will get to its destination? Wired networks are pretty reliable because there's a solid wire taking the messages from one computer to another. But wireless networks, particularly phone networks can be unreliable due to bad signal strength, walls and other obstacles getting in the way of the signal or too many people trying to use them at the same time. So, networks need a physical infrastructure that links computers either by wires or ways that lets them send signals to each other. But that isn't enough to enable communication. It isn't enough just to send messages. Computers need to be able to understand the messages that are being sent to them. For that, they need a language to communicate in. The languages used for network communication are called protocols. Protocols aren't like human languages. To be understood by computers, they have to follow very strict rules. A protocol defines the rules that say how you communicate, when you send messages and what they contain. If two computers follow the same protocol, they can understand each other. If they don't, the communication will go wrong. So, there are many different types of network that can vary both in the physical medium they used for communication and the protocols that are used to communicate on that medium. Take for example wireless networks such as Wi-Fi and cellular networks, the networks we use to send data from mobile phones. They use the same basic technology. They are both wireless and based on electromagnetic waves, but they're different in the details. There are physical differences in the technology. Cellular networks are much longer range than Wi-Fi, and generally have lower bandwidth. They also use different protocols. So, although your phone might support both, it needs to use different technologies to do so. Networks are meant to connect computers. But having different types of network is a major obstacle if computers on one network can't communicate with computers using a different type of network. That is where the Internet comes in.